[775] THE MOVEMENT OF SEA-URCHIN SPERMATOZOA

Although the mechanical activity of spermatozoa plays an essential role in the process of fertilization there is little precise information concerning the type and speed of movement of individual cells in their normal state of activity. This paper represents an attempt to fill this gap with respect to a sea-urchin whose spermatozoa, being of the typical monoflagellate type, provide valuable material for the study of flagellar propulsion in general. Accurate observation of the movements of fully active spermatozoa encounters three main difficulties: first, the diameter of the tail—on whose activity the pro-pulsion of the head depends—is near the limit of resolution by transmitted light; secondly, the normal frequency of the tail's movements is far above that which can be followed by the human eye; thirdly, direct observation is almost entirely restricted to cells constrained to move in the plane of the microscope's field by the presence of a glass/water or water/air interface. The motion of cells moving freely in a bulk of fluid is extremely difficult to observe. Proximity to a surface not only affects the freedom of the cell to roll or pitch about its own axes, but also expose